Diagnostic radiopharmaceuticals for localization in target tissues exhibiting a regional pH shift relative to surrounding tissues

A radiopharmaceutical method for observing at least one target tissue or organ of a mammal, the target tissue or organ having a physiologically significantly lower intracellular pH than the blood of the mammal, the method comprising the steps of:

• introducing a radiopharmaceutical compound into the bloodstream of the mammal, the compound containing at least one radioactive element, other than an isotope of iodine, emitting .gamma., X-ray or positron radiation detectable by radiation detecting means, the compound having a n-octanol-aqueous medium partition coefficient which is significantly dependent on pH of the aqueous medium at least in the pH range of 6.6 to 7.6 so that the n-octanol-aqueous medium partition coefficient of the compound increases with increasing pH at least in said pH range;
• said compound containing at least one amine group;
• allowing the compound to penetrate into cells of the target tissue or organ by diffusion through the cell membranes wherein due to the lower pH, the n-octanol-aqueous medium partition coefficient of the compound is significantly diminished whereby a rate of exit of the compound from the target tissue or organ by diffusion is significantly diminished compared to its rate of entry into the target tissue or organ, resulting in at least temporary accumulation of the compound in the target tissue or organ;
• and
• detecting with a radiation detecting means the accumulated compound in the target tissue or organ.